1. Technical Field of the Invention
The present invention relates to the field of battery charges, and, in particular, to circuitry for terminating the charge process of a battery charger.
2. Description of Related Art
Battery chargers for common Nickel Cadmium (NiCd) cell batteries and Nickel-metal-hydride (NiMH) cell batteries employ several different techniques for terminating the charge process of a battery or for a group of batteries being charged simultaneously. One technique is to terminate the charging of a battery when the battery reaches or exceeds a predefined voltage threshold or displays a specified voltage curve characteristic. Another technique involves merely charging the battery for a specified amount of time.
A problem with merely charging a battery for a specified period of time is that it is difficult to pick one time period which will serve to properly charge all batteries. Sometimes, batteries can be charged too long. In such a case, an overcharged battery can be damaged from the build up of excessive internal temperatures. To avoid this problem, a lower time period should be used. However, a related problem then is that some batteries will not be charged a sufficient amount.
Because the approach of merely charging a battery for a specified period of time is overly simplistic and generally inadequate, other approaches have been developed to terminate the charge process of a battery. One such approach is known as minus delta V termination (MDVT). MDVT processes measure the voltage across the battery's terminals until a peak voltage has been obtained. Once a charger determines that the voltage across the battery terminals is less than a peak value (i.e., there is a negative slope to the voltage charge curve), the charge process is terminated. One problem with the MDVT process, however, is that the negative voltage slope can become undetectable some times. For instance, when a battery is operating at an elevated temperature, the negative voltage slope is frequently undetectable.
Other types of batteries cannot be charged by a charger that uses MDVT termination. For instance, lithium-ion, rechargeable alkaline, lead-acid and Rechargeable Alkaline Manganese (RAM) batteries do not display a negative voltage slope after reaching a maximum charge. Accordingly, there currently exists a need for different types of chargers for these batteries, which renders existing and common battery chargers mostly useless in this regard. As the need for higher capacity and lighter weight batteries increases, the proliferation of these newer battery types will also increase. This, in turn, will make perfectly good MDVT chargers obsolete. Moreover, the chargers for these newer types of batteries may become more complex and expensive as new methods are developed to terminate a charge process. There is a need, therefore, for batteries to be compatible with chargers that use a minus delta V termination process.